1. Field of the Invention
The present disclosure generally relates to image processing and, more particularly, to a white balance control technique applied to image data.
2. Description of the Related Art
Image capture apparatuses which use an image sensor, such as digital cameras and digital video cameras, have a function called a white balance control function. This function is used for adjusting a color tone of captured image data. There are two types of white balance control, which include manual white balance control and automatic white balance control. According to the manual white balance control, first, an image of a white color object is captured. Then, a white balance coefficient is calculated based on the image, and the obtained calculated white balance coefficient is applied to the entire screen. According to the automatic white balance control, a portion which is assumed to be white is automatically detected from the captured image data. Then, the white balance coefficient is calculated based on a mean value of color components of each color, and the obtained white balance coefficient is applied to the entire screen.
According to the conventional automatic white balance control, if a flash unit flashes in a scene where alight source other than the flash unit exists, a white balance coefficient calculated as described above is applied to the entire screen. Thus, it is difficult to perform the white balance control considering the tint of each light source. For example, if the environment light includes light from a low color temperature light source such as an incandescent light source, a white balance that matches the low color temperature light source cannot be obtained if the white balance is adjusted for the flash light. This is because the flash unit is a high color temperature light source. On the other hand, if the white balance is adjusted for the low color temperature light source, a white balance that matches the flash light cannot be obtained. Further, if the white balance control is performed so that the white balance is adjusted for a medium between the low and the high temperature light sources, since the white balance corresponds to neither of the light sources, the tint of the area lighted by the flash light will be bluish and the area lighted by the low color temperature light source will be reddish.
Under such circumstances, Japanese Patent No. 3540485 discusses a technique useful for controlling the white balance. According to this technique, image data which is captured at the light emission timing of the flash unit is compared with image data which is captured at the non-emission timing of the flash unit for each arbitrary object area and a ratio of the data is obtained. Further, a contribution ratio of the flash light is determined according to the value of the ratio. Then, the white balance control is performed using a white balance control value selected for each object area according to the determined contribution ratio.
However, according to the technique discussed in Japanese Patent No. 3540485, since the development processing is performed after the white balance control value is selected for each area, control such as color reproduction control may not appropriately match the white balance control value.
Further, if the technique discussed in Japanese Patent No. 3540485 is used, there is a time lag between the image data captured at light emission timing of the flash unit and the image data captured at non-emission timing of the flash unit. Thus, when the object is moving or the image capture apparatus itself is moving, a detection error regarding the flash coverage area or the amount of flash light may occur. Accordingly, a shift in tint is likely to occur at the boundary portion of the object. Especially, when an image is captured at high-power zoom, since even a slight motion of the image capture apparatus causes the detection errors of the flash coverage and the amount of flash light, the shift in tint is more likely to occur.